Airway inflammation characterize's a number of severe lung diseases including asthma and chronic obstructive pulmonary disease (COPD). Events leading to airway obstruction include edema of airway walls, infiltration of inflammatory cells into the lungs, production of various inflammatory mediators and increased mucous production. The airways of asthmatic patients are infiltrated by inflammatory leukocytes, of which the eosinophil is the most prominent component. The magnitude of asthmatic reactions is correlated with the number of eosinophils present in the lungs. The accumulation of eosinophils is found dramatically in the lungs of asthmatic patients, which are capable of lysing and activating cells and destroying tissues. Upon activation eosinophils release inflammatory cytokines such as IL-1, IL-1β, IL-3, IL-6, IL-8, IL-12, TNF-α and inflammatory mediators such as PAF (Platelet-Activating Factor), LTD4 (Leukotriene-D4) and relative oxygen species that can produce edema, bronchoconstriction. TNF-α is involved in the pathogenesis of a number of autoimmune and inflammatory diseases (Bartolome Celli, Chest, 2006, vol 129 no. 1, 5-6). Consequently, manipulation of the cytokine signaling or biosynthetic pathways associated with these proteins may provide therapeutic benefit in disease states mentioned above. It has been well demonstrated that TNF-α production in pro-inflammatory cells becomes attenuated by an elevation of intracellular cyclic adenosine 3′,5′-monophosphate (cAMP); this second messenger is regulated by the phosphodiesterase (PDE) family of enzymes. The phosphodiesterase enzymes play an integral role in cell signaling mechanisms by hydrolyzing cAMP and cGMP to their inactive 5′ forms. Inhibition of PDE enzymes thus results in an elevation of cAMP and/or cGMP levels and alters intracellular responses to extra cellular signals by affecting the processes mediated by cyclic nucleotides. Since eosinophils are believed to be a critical proinflammatory target for asthma, identification of the expression of PDE4 gene family in eosinophils led to the PDE4 as potential therapeutic target for asthma [Rogers, D. F., et al., Trends Pharmacol. Sci., 1998, 19, 160-164; Barnes, P. J., Trends Pharmacol. Sci., 1998, 19, 415-423, L. Pages, et. al., Expert Opin. Ther. Patents 2009, 19, 1501-1519].
Phosphodiesterase type 4 (PDE4) is cAMP-specific and Ca2+ independent enzyme and hydrolyses cAMP in mast cells, basophils, eosinophils, monocytes and lymphocytes. The association between cAMP elevation in inflammatory cells with airway smooth muscle relaxation and inhibition of mediator release has led to widespread interest in the design of PDE4 inhibitors [Trophy, T. J., Am. J. Respir. Crit. Care Med., 1998, 157, 351-370; P. J. Barnes, Eur Respir Rev 2005, 14: 94, 2-11; Wolfgang Bäumer et al., Inflammation & Allergy—Drug Targets, 2006, 6, 17-26; Joseph P. Grande et al., Exp Biol Med 2007, 232, 38-51]. Excessive or unregulated TNF-α production has been implicated in mediating or exacerbating a number of undesirable physiological conditions such as diseases including osteoarthritis and other arthritic conditions; septic shock, endotoxic shock and respiratory distress syndrome. Since TNF-α also participates in the onset and progress of autoimmune diseases, PDE4 inhibitors may find utility as therapeutic agents for rheumatoid arthritis, multiple sclerosis and Crohn's disease. [Nature Medicine, 1995, 1, 211-214 and ibid., 244-248]. TNF-α is also reported to be a factor of insulin-resistant diabetes because it declines the phosphorylating mechanism of insulin receptors of muscle and fat cells [J. Clin. Invest., 1994, 94, 1543-1549].
It has been demonstrated that increasing cAMP levels within these cells results in suppression of cell activation, which in turn inhibits the production and release of pro-inflammatory cytokines such as TNF-α. Since eosinophils are believed to be a critical pro-inflammatory target for asthma, identification of the expression of PDE4 gene family in eosinophils led to the PDE4 as potential therapeutic target for asthma. The IL-6, tumor necrosis factor TNF-α, E-selectin, and nitric oxide (NO) production have been reported to be involved in the pathogenesis of LPS-induced uveitis, PDE4 inhibitors are known to suppress cutaneous inflammation and LPS-induced TNF-α expression including IL-6 inhibition, forming a viable strategy for treatment of uveitis (Investigative Ophthalmology and Visual Science, 2004, 45, 2497-2502).
Interest in the drugs capable of selective inhibition of PDE4 has taken much attention due to several factors: (a) tissue distribution of PDE-4 strongly suggested that the pathologies related to the central nervous and immune systems could be treated through the selective PDE4 inhibitors (b) increase in intracellular cAMP concentration, the obvious biochemical consequence of PDE-4 inhibition, has been well characterized in immuno-competent cells where it acts as a deactivating signal.
Four human cDNA isoforms of PDE-4 (PDE4-A, B, C and D) were identified. mRNA for all these four isoforms was expressed in the human lungs. PDE4-A, B, C and D were expressed in eosinophils. Of these gene families, PDE-4 characterized as the cAMP-specific gene family has been shown to predominate in proinflammatory human lymphoid and myeloid lineage cells.